Transport of bulk powders from the manufacturing location to user location often pose problems particularly for powders that are by nature cohesive.
To transport bulk powders economically, they are typically shipped in metric ton bags commonly called semi-bulk containers (SBC's) or large bulk containers commonly called COFC's (container on railway flat car) or IMC's (intermodal containers). But, these alternatives have associated problems that increase transportation and handling costs, particularly for cohesive powders.
Use of SBC's offer efficiencies if the end user is willing to accept the SBC for direct use in their operations. If the end user prefers bulk delivery, the SBC must be dumped into large storage containers or into bulk trucks for delivery or use. This operation requires time and potentially represents some loss of product due to the incomplete emptying of the bag. The residual material (known as a heel) is often discarded with the bags.
IMC's may be used to efficiently transport bulk powders by rail or by sea, but on receipt at the receiving location the bulk powder must be transferred to storage or to bulk trucks for delivery. Economical unloading of bulk materials from IMC's at a transfer terminal or end user must be accomplished rapidly (ideally in one or two hours) in order to make effective use of labor and the expensive capital equipment needed to handle or unload the IMC. Any connection between the IMC and an unloading system must be made (and later removed) rapidly in order to minimize the impact on the total unloading cycle time. Spillage or airborne dust is not permissible due to product losses and environmental and housekeeping concerns.
Free flowing bulk materials such as plastic pellets and agricultural grains can easily be unloaded from IMC's containing 20 tons or more of material. Cohesive powders, on the other hand, are extremely difficult to unload from IMC's due to their bulk handling properties. These properties fall into four categories—dustiness, wall friction, gas permeability, and cohesive strength.
Discharging a load of cohesive bulk powder requires long times and presents unloading problems even when gravity unloading is assisted by the extensive use of vibrators or the use of pneumatic aeration systems. Such assisted unloading methods generally lead to dusting problems. Often, in spite of much effort directed to discharging all the powder stored in the IMC, substantial heels remain in the liner unavailable for use.
U.S. Pat. No. 3,999,741 teaches a method of unloading pigments from a bulk container by adding liquid to the container and removing the pigment as dispersion.
German Patent Publication DE 34 29 167 A1 teaches a method and apparatus for unloading a flexible container (such as a big bag) by placing the container on a vibrating element.
U.S. Pat. No. 4,781,513 teaches an apparatus for unloading and spreading a bulk material such as asphalt over the ground.
U.S. Pat. Nos. 4,875,811; 5,096,336; and 5,378,047 teach related inventions. In each case the invention is directed to unloading a bulk container using a pneumatic conveying apparatus. In the apparatus and process taught in these patents, bulk material such as polycarbonate resin is directed through a flexible conduit to a rotary valve that feeds the particulate material into a lower hopper for pneumatic conveying into a suitable storage facility.
U.S. Pat. No. 4,301,943 teaches a container and process to unload melamine powder from a bulk container. According to this patent melamine powder is unloaded through a discharge device having a hopper portion, a connector portion and a rotary pump assembly wherein certain hopper dimensions of angle, height and opening diameters are required.
The present invention provides an unloading system that is fully effective even with cohesive bulk powders such as pigmentary titanium dioxide.